Many farm implements are provided with various attachments to improve their operational efficiency, enhance their versatility and/or augment their mobility. The location and physical securing of these attachments to the implement often results in a non-uniform distribution of weight along the implement. A plurality of hydraulic rams are often required in order to raise and position the implement in order to utilize conventional cylinder diameters at acceptable levels of hydraulic pressures. Those rams or cylinders carrying the smaller weight in a non-uniformally distributed load will extend first and raise the implement ahead of the more heavily loaded rams. Such action is generally not desirable.
Master-slave arrangements in which the hydraulic cylinders are connected in series, the fluid expelled from the master cylinder being utilized to synchronize the movement of the slave cylinder with the master, or spool type proportional flow dividers have been utilized in the past to achieve a level lift of the implement. The master-slave arrangements require different size cylinders, may cause distortion or deformation of the implement frame and require piston seals to pass over an orifice to achieve rephasing. Spool type proportional flow divider systems characteristically exhibit wide volumetric variations between work ports as a function of flow and pressure at each port, are limited in how many cylinders can be synchronized and require an interconnection between work ports through a controlling orifice which allows flow from a heavily loaded cylinder to transfer to a less heavily loaded cylinder upon reaching the desired position. Both master-slave arrangements and spool type flow dividers systems may require manual venting or bleeding to rephase the cylinders.
The use of a rotary flow divider to achieve level lift of a farm implement and rephasing to synchronize the lift cylinders is disclosed and claimed in U.S. patent application Ser. No. 156,892, filed June 5, 1980, entitled "Implement Level Lift System" by H. J. MacKenzie, now U.S. Pat. No. 4,324,411. This system relies upon leakage around the rotors of the rotary flow divider to achieve rephasing. While such an arrangement will operate, the time required to complete rephasing is a function of the amount of such leakage, the fluid pressures involved and the volume of fluid required to bring the cylinders into phase.
It is, therefore, an object of this invention to provide a hydraulic system for an implement which maintains a relatively level attitude for the implement as it is raised and which will efficiently and quickly achieve rephasing when necessary to synchronize the lift cylinders.
It is also an object of this invention to provide such a level lift system for an implement in which rephasing is accelerated by an increase in the flow of hydraulic fluid to the cylinder which is lagging behind.
It is another object to provide such a level lift system which automatically rephases rapidly, minimizes the need to vent or bleed the system, which utilizes conventional cylinders or rams, and which generally obviates the deficiencies of prior art systems.